Segment Information (Net Premiums Earned by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Net premiums earned	$ 544.9	$ 580.1
Operating segments | North American Insurance
Segment Reporting Information [Line Items]
Net premiums earned	295.0	316.3
Operating segments | North American Insurance | General casualty
Segment Reporting Information [Line Items]
Net premiums earned	92.1	96.7
Operating segments | North American Insurance | Professional liability
Segment Reporting Information [Line Items]
Net premiums earned	81.6	101.5
Operating segments | North American Insurance | Programs
Segment Reporting Information [Line Items]
Net premiums earned	47.6	39.6
Operating segments | North American Insurance | Other specialty
Segment Reporting Information [Line Items]
Net premiums earned	32.6	25.2
Operating segments | North American Insurance | General property
Segment Reporting Information [Line Items]
Net premiums earned	27.3	33.8
Operating segments | North American Insurance | Healthcare
Segment Reporting Information [Line Items]
Net premiums earned	13.8	19.5
Operating segments | Global Markets Insurance
Segment Reporting Information [Line Items]
Net premiums earned	92.7	94.2
Operating segments | Global Markets Insurance | General casualty
Segment Reporting Information [Line Items]
Net premiums earned	21.3	23.6
Operating segments | Global Markets Insurance | Professional liability
Segment Reporting Information [Line Items]
Net premiums earned	23.0	23.5
Operating segments | Global Markets Insurance | Other specialty
Segment Reporting Information [Line Items]
Net premiums earned	30.9	30.8
Operating segments | Global Markets Insurance | General property
Segment Reporting Information [Line Items]
Net premiums earned	17.5	16.3
Operating segments | Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	157.2	169.6
Operating segments | Reinsurance | Property
Segment Reporting Information [Line Items]
Net premiums earned	76.6	84.4
Operating segments | Reinsurance | Casualty
Segment Reporting Information [Line Items]
Net premiums earned	44.2	46.2
Operating segments | Reinsurance | Specialty
Segment Reporting Information [Line Items]
Net premiums earned	$ 36.4	$ 39.0